I think the treatment has generally been appropriate to each edition (with the exception of Greg Bell's work in the little brown books).
That Elmore piece screams "Second Edition AD&D!" to me, from the Thomas Kinkade-ish style to the "posing for a photo" conceit. The "Rock of Bral" and "Ivory Triangle" covers also stand out as period pieces.
The "Dune Trader" cover, on the other hand is very Dark Sun, and (without the copy) the painting could as easily evoke Gamma World or Skyrealms of Jorune. That in itself is rather 2nd Edition-like, coming from a period in which the game's own identity was seen as inadequate and so old bits got endlessly recycled in mixture with elements borrowed from other original creations. The results in my opinion lacked the integrity and vigor of the sources they cannibalized, because they lacked the "fire in the belly" of a creator with a singular vision (even as wildly eclectic a vision as, say, Dave Hargrave's). Not only was TSR's AD&D no longer stamped with Gary Gygax's personality, it nor longer bore any stamp but that of a committee.
I see that what I've written so far focuses on negatives. The differences of Spelljammer, Dark Sun, and so on from what had become viewed as a "standard" D&D setting reflected the renewed (but commercialized) emphasis in 2E on the D&D game's wide-open possibilities. The flood of kits and other options tended toward rules-heaviness, but also offered a lot of inspiration for the making of "a game of one's own" that had been central to the original rules-light release.
The very polished and self-consciously commercial presentation reflects something that was both good and bad: the line drawn between "game professionals" and "hobbyists." The nature and sheer volume of "support" provided by the former enabled the latter to embrace a wider demographic. People who wanted to could still "do it yourself", but there was plenty for those with more cash to spare than time and energy. Volumes were published covering in depth (and accessible prose) every aspect of Dungeon Mastering, often reduced to easy-to-follow recipes. The growing disconnection between producing "product" for publication and actually running a campaign was perhaps not so healthy, at least from the perspective of those who appreciate the merits of more rough hewn works clearly derived from the latter context.
The "iconic" artists of 1st ed. AD&D (Sutherland, Trampier, Otus) were strongly associated with the players/designers of the game itself. Their depictions of (for instance) kobolds sometimes became more definitive than the original concepts -- because they somehow "felt right." Black and white ink line work was a practical necessity, but also a medium conducive to drawing connections with sources of inspiration for the game. (At least two illustrations in the 1st ed. DMG are based on works by artists rather better known in academia.)
Figures on average are not as "posed" as in later editions, and indeed often catch the eye only after the scenery that is so much neglected these days. The adventurers tend to look less like superheroes, more like hard-bitten soldiers of fortune. Outside of Otus, their kit draws inspiration from actual medieval artifacts -- and the fantastic of Otus is unmistakably his own. What's "cool" seems a lot less associated with the "heroes" looking good; indeed, misfortune is a fairly frequent (and often humorous) subject.
There's a rough, sometimes amateurish aspect that may at times obscure the artists' real technical competence, but is in keeping with the game's general tenor. It does not want to be another "glitzy" Disco Era commodity. Its heart is made of Weird Tales pulp.
The Moldvay and Mentzer Basic sets pretty much kept up with trends in the AD&D line. Some pieces may be timeless, but on balance the books are well situated in the 1980s. One can see in them the transition from Otus to Elmore, the embracing of which may suggest a reason they are often called "Classic" D&D.
With 3E, we get into a whole new set of influences all around. "Who the heck is Abe Merritt, and what has he got to do with D&D?" is the sort of question that I think was often not even in the minds of the new generation. Hannes Bok was probably about as much "off the radar" for the illustrators.
I see 4E as a fuller realization of that new ethos. Perhaps it is merely as much "of its time" as the others. To me it seems more limited in its horizons, disconnected from the long legacy that made it possible -- but I think it takes a remove of more time to get a critical perspective.
Not much is really to my taste in terms of subject matter and composition, but I can appreciate many employments of technique. The graphic design as a whole I find much easier on the eyes than the 3E books, and in general I like the illustrations better. The big ones I rank weaker than the occasional pieces, which include some gems from artists of whose work I would be delighted to see more.